This invention relates to medical devices and more particularly to an improved ultrasonic nebulizer for administering a medicament aerosol to a patient.
Ultrasonic nebulizers are commonly used for administering medicament aerosols to patients suffering from respiratory diseases such as emphysema. Ultrasonic nebulizers generally comprise a ceramic piezoelectric transducer located at the bottom of a liquid reservoir and a radio frequency power supply for exciting the transducer. When the transducer is excited by alternating current having a frequency at or near the resonant frequency of the transducer, the transducer vibrates to produce a geyser above the surface of the liquid in the reservoir and extremely fine liquid particles from the geyser. Air inhaled by a patient is drawn past the geyser to entrain the fine liquid particles for delivering the medicament to the patient.
In hospitals and clinics, relatively large ultrasonic nebulizers are commonly used. These nebulizers are placed on tables or are mounted on a floor stand. For patients requiring quantitative administration of medication, smaller portable nebulizers are desirable. In one embodiment, a hand-held nebulizer is provided with an internal power supply which generates radio frequency current. A measured dose of medicament is placed in the nebulizer for administering to the patient. However, considerable problems have occurred in prior art ultrasonic nebulizers of this type. In one prior art hand-held ultrasonic nebulizer, a trigger or finger actuated switch has been provided on the nebulizer for manually energizing the transducer during inhalation. However, since an aerosol is not generated unless the switch is closed, some of the inhaled gas will not include medication if the patient does not properly time the switch closure with inhalation. In another embodiment, the power supply may be connected to continuously energize the transducer. In both types of ultrasonic nebulizers, there is a strong possibility of damaging the transducer by allowing the transducer to operate in a mechanically unloaded or dry condition. This is particularly true since it is desirable to operate the nebulizer until dry to administer an entire measured dose of medicament to the patient and also because the patient may at some point tip the hand-held nebulizer, allowing the liquid medicament to flow away from the transducer. The larger size nebulizers which are mounted on a stand or supported on a table are provided with liquid level sensing switches which shut off the nebulizers when the liquid drops below a predetermined level.
Typical prior art ultrasonic nebulizers include a radio frequency power source which is conventionally coupled to the piezoelectric crystal transducer. This arrangement delivers full power to the transducer when it is mechanically loaded or covered with a liquid and also when it is not covered with a liquid and is mechanically unloaded. If the transducer is allowed to run dry or become mechanically unloaded, it will be damaged or destroyed if power is continuously applied.